Volcaniclastic rocks and volcaniclastic facies in the Middle Precambrian (Aphebian) Belcher Group, Northwest Territories, Canada

Abstract

The Middle Precambrian Flaherty Formation consists of a thick sequence of volcanic rocks of both effusive and explosive origin, derived from a source area west of the Belcher Islands. The effusive volcanic facies are made up of thick, massive lava flows, some of which are compound, that are intercalated with thick lenticular units of pillowed lava. Facies of explosive origin, the volcaniclastic rocks, are a product of phreatic and phreatomagmatic eruptions. A variety of volcaniclastic facies are recognized on the basis of bed geometry, sedimentary structures, and textural characteristics, and include: thinly bedded tuffs and lapillistones that were deposited by fallout from vertical ash columns (some tuffs contain accretionary lapilli and hence originated from subaerial eruption columns); pillow talus deposits (hyaloclastites) resulting from the rapid quenching of lava in water; volcaniclastic turbidites representing relatively deep-water resedimented tephra; and a single massive pyroclastic flow that was emplaced during a single depositional event, and may have been derived from a Plinian eruption or series of Surtseyan eruptions. Systematic variations in these volcaniclastic facies, for example an increase in the proportion of turbidite units towards eastern Belcher Islands, indicate that water depth increased eastward.Based on physical volcanological aspects, the Flaherty Formation is compared with some modern and ancient analogues. The style of volcanism envisaged for the Flaherty is similar to that of Iceland, or possibly to an immature stage of island arc development. From a tectonic viewpoint, the comparison with Iceland is problematical because it is part of an oceanic regime, whereas the Flaherty Formation is more closely related to an ensialic regime. Direct comparison with an island arc also is difficult because little is known about volcanic facies relationships in the early growth stages of such arcs.